Paleoecology Blogs

“Pollen richness, a cautionary tale” is out in the Journal of Ecology

I have a new paper out in the Journal of Ecology here. The article also contains a supplement with R Markdown code that will allow users to reproduce the analysis in the article nearly faithfully (here, and as a code repository on GitHub here if you want to fork it and modify the code).

Figure 5 from Goring et al., the relationships between plant richness and smoothed pollen richness and vice versa both show a slightly negative relationship (accounting for very little variability), meaning higher plant richness is associated with lower pollen richness.

The paper sets out to test whether pollen richness is truly a measure of plant richness in a modern context in British Columbia. Using a network of 16,000 plant sample plots and 167 lake sediment records from across British Columbia we show that pollen cannot be counted on to faithfully reproduce plant richness in this region. We show that rarefaction plays little role in the lack of a relationship between plant species richness and pollen taxonomic richness, and that the taxonomic smoothing between plant species and the representative pollen morphotypes does degrade the relationship, but that some signal still exists.

Ultimately we attribute the lack of any relationship between pollen richness and plant species richness to the process of pollen production, transport, deposition and preservation (taphonomy). We’ve known for some time now that different pollen taxa are transported different distances from their parent plants. For example, Rousseau et al. (2008) found a number of pollen taxa in aerial traps in Greenland that represented very long range pollen transport and complex transport mechanisms for air packets to southern Greenland. Havinga did some interesting long term experiments in pollen preservation, culminating in his 1984 paper in Pollen et Spores [citation] that showed differential preservation in pollen taxa. The Havinga paper is hard to come by (with no online PDF anywhere), but it’s a great paper and deserves to be more widely cited.

I’m really happy with the paper, partly because I’m confident in the results for British Columbia, but also because I think it speaks to an interesting issue. We do see patterns of change in pollen richness through time, most spectacularly in Jaramillo et al. (2006), but if our results in British Columbia hold true everywhere, then maybe we’re not actually measuring plant diversity. There are reasons to be skeptical of this though: (1) Records from a single (or multiple closely related) investigator(s) can achieve a higher level of taxonomic resolution than meta-analyses such as the one in our paper. (2) Different regions may show different proportions of aerial and insect pollinated taxa, and overall pollen diversity due to the phylogenetic signal of pollen morphology.

A number of pollen records in British Columbia show significant changes in pollen richness during the Holocene using GLMs or GAMs (calibrated years BP on the x-axis, pollen richness on the y-axis).

All in all, the paper is titled “A cautionary tale” because I believe that the link between pollen diversity and plant diversity is something that hasn’t been explicitly investigated, but has broad implications for the conservation applications of paleoecology in the future. I am hopeful that this work can be repeated in other regions with both highly (taxonomically) resolved datasets, such as the one published in Salonen et al. (2012) and generalized datasets such as the North American Pollen Dataset where pollen taxonomy is coarsened to a somewhat lower standard (we implemented the NAMPD taxonomy used in Whitmore et al., 2005; pdf). I’m particularly curious in what patterns tropical regions might show us.

The same issue also has a paper (here) with Jacquelyn Gill, looking at validating the link between megaherbivore grazing and Sporormiella abundance that underlies the use of the fungal spore as a proxy for megaherbivore decline in the late-Glacial as indicated by earlier papers (Gill et al., 2009, 2012). These papers share a common thread in having been partly inspired by Steve Jackson’s excellent 2012 QSR paper, “Representation of flora and vegetation in Quaternary fossil assemblages: known and unknown knowns and unknowns”. We need to understand the patterns we see in the paleoecological record, and we need to test them explicitly (John Birks has long advocated explicit hypothesis testing in paleoecology). Without hypothesis testing paleoecologists continue to tell stories, but fail to make the stories applicable to other disciplines. Our society is faced with a future that is difficult to understand through any single discipline, and paleoecology lends an ecological perspective that simply cannot be captured by any other ecological discipline. If we want to make this data and theory useful to other researchers, we must make it robust.